1. Field of the Invention
The present invention relates to non-reciprocal circuit devices for use in a high-frequency band like a microwave band (for example, isolators and circulators), and in particular, to non-reciprocal circuit devices capable of meeting size and price reduction requirements for use in mobile-communication equipment.
2. Description of the Related Art
In general, non-reciprocal circuit devices such as lumped-constant isolators and circulators have characteristics in which the amount of attenuation is extremely small in the direction along which a signal is transmitted, and is extremely large in the reverse direction.
The isolators of this type include a conventional isolator having, for example, a structure as shown in FIG. 7. This isolator includes a magnetic assembly 5 consisting of a permanent magnet 3, three central conductors 51, 52 and 53 and a ferrite member 54, and a resin case 7, both disposed in a closed magnetic circuit formed mainly by a upper yoke 2 and a lower yoke 8. The ports P1 and P2 of the central conductors 51 and 52 are connected to input/output terminals 71 and 72 and matching capacitors Co which are formed in the resin case 7. The port P3 of the central conductor 53 is connected to another matching capacitor Co and a terminal resistor R. One end of each capacitor Co and one end of the terminal resistor R are connected to ground terminals 73.
FIG. 8 shows an equivalent circuit diagram of the above-described isolator. As shown in FIG. 8, the conventional isolator has a structure formed such that the matching capacitors Co are connected to the ports P1, P2 and P3 at the heads of the central conductors 51, 52 and 53, respectively, to form matching circuits, and the terminal resistor R is connected to one port P3. Inductors L have equivalent inductance formed by the ferrite member 54 and the central conductors 51, 52 and 53.
The above-described isolator is employed in the transmission/reception circuit unit of a shared antenna circuit used for mobile-communication apparatuses such as cellular phones and automobile telephones. As shown in FIG. 9, the isolator is used when mounted on the front surface of a mounting base-member 10 where input/output transmission lines 11 and 12, and ground terminals 13 are formed, with a ground electrode formed on almost the entire back surface.
In general, a non-linear characteristic exists in an amplifier built into such a communication apparatus, which causes unnecessary radiation, namely, spurious radiation (a multiple of a fundamental wave, in particular, the second harmonic and the third harmonic). The unnecessary radiation may cause interference, and may also cause a malfunction of a power-amplifying unit in another communication apparatus. Accordingly, the unnecessary radiation is standardized to a constant level or less.
In addition, since the isolator has a bandpass-filter function in its transmission direction, it characteristically has a large amount of attenuation in a frequency band away from its pass band. However, isolators are originally not intended for obtaining attenuation outside their pass band. Thus, the desired amount of attenuation in the frequency bands (particularly, the second and third harmonics of the fundamental wave) where the unnecessary radiation occurs cannot be obtained by the above-described conventional isolator. Accordingly, this type of conventional communication apparatus employs a method for attenuating the unnecessary radiation with a filter or the like.
In other words, the use of the above conventional isolator requires an unnecessary-radiation prevention filter, and causes a problem in which the cost of components increases due to the prevention filter, and a problem in which the demand for size and price reduction cannot be met.
In addition, in general, the characteristic impedance of input/output transmission lines on a mounting base-member is set to 50 .OMEGA.. However, in one modern type of small-sized communication apparatus, for example, a very thin mounting base-member 0.1 to 0.5 mm thick is used, and the width of a transmission line needs to be less than 1 mm in order for the impedance of the transmission line to be 50 .OMEGA.. Such a narrow line width cannot reserve a sufficient area for soldering, which makes it difficult to mount the isolator using an automatic mounting apparatus and to obtain sufficient mounting strength (soldering strength). Thus, as shown in FIG. 9, on the mounting base-member 10 that is actually used, the soldering lands 11a and 12a are formed as wider portions of the transmission lines 11 and 12, to be soldered to the input/output terminals 71 and 72 of the isolator.
In this arrangement, electrode-distribution parasitic capacitors Cp formed by the soldering lands 11a and 12a greatly shift the characteristic impedance of the transmission lines 11 and 12 from 50 .OMEGA., which makes it impossible to perform impedance matching with the isolator, so that the operating central frequency of the isolator decreases disadvantageously. For countermeasures, the conventional communication apparatus uses an isolator whose operating central frequency is set too high, or employs a complicated and expensive method in which an inductor which resonates in parallel with the electrode-distribution capacitor Cp at the operating central frequency is formed on the mounting base-member 10 so that the value of the electrode-distribution capacitor Cp is canceled. In other words, when the conventional isolator is used, it is necessary to produce various modified isolators to match with the electrode-distribution capacitor Cp, or to form an inductance on the mounting base-member 10 for canceling the value of the electrode-distribution capacitor Cp.
In addition, the input/output impedance of the isolator and the characteristic impedance of the transmission lines on the mounting base-member are generally around 50 .OMEGA.. There may be however a case in which the characteristic impedance of the transmission lines is set to a value different from 50 .OMEGA. in accordance with an amplifier to be used and the interconnection pattern of the mounting base-member, so it may be demanded that the input/output impedance of the isolator be set to a value different from 50 .OMEGA., for example, 60 .OMEGA..
According to the above-described conventional isolator, a change of its input/output impedance requires a change (re-design) of each component (such as a central conductor and a matching capacitor) included in the non-reciprocal circuit device. Accordingly, various types of components in accordance with the input/output impedance are needed, which disadvantageously increases the cost of components, the costs of component control, and production costs.
To this end, it is a feature of the present invention to provide a non-reciprocal circuit device which is capable of having an increased amount of attenuation outside its pass band, having improved mounting strength, and whose input/output impedance can be easily changed and set to a desired value, which thus contributes to a reduction in size and price.